InterNACHI Septic common mistakes
InterNACHI Septic · Understand the traps and how to avoid them.
Treating the septic tank as a treatment device and describing effluent leaving it as 'clean' or 'treated', rather than describing it as clarified but still pathogen-rich settled sewage that relies on the drainfield soil for final treatment.
Homeowner-level sources and marketing language blur the tank and the soil absorption system, so candidates memorize 'the tank treats the waste' and then answer component-function items incorrectly.
What to do instead
Fix the model in two sentences you can recite: the tank only separates (scum floats, sludge settles, anaerobic digestion reduces volume) and stores solids; the drainfield soil does the biological treatment and disposal. Practice labeling a three-compartment sketch - inlet, baffles, outlet - and stating what each does.
Confusing the roles of the inlet and outlet baffles, or asserting that a missing outlet baffle is a minor cosmetic issue, when a deteriorated outlet baffle is a direct route for scum to escape into the drainfield.
Both baffles look like similar pipes, and candidates who have only seen a tank from the top cannot visualize the scum layer depth relative to the outlet tee.
What to do instead
Memorize the failure logic: the outlet baffle/tee draws from the clear middle zone, so when it is missing or rotted off, the scum layer migrates to the absorption area and clogs it. Learn to state that inlet baffles protect the inlet and reduce disturbance, while the outlet device protects the drainfield - and that a missing outlet baffle is a reportable defect recommending evaluation and likely replacement.
Judging soil suitability from surface appearance or a single hand-texture squeeze, without addressing depth to a limiting layer (bedrock, seasonal high water table, fragipan) or the percolation/loading evidence the local authority requires.
Candidates apply a gardening mindset ('it looks like good dirt') and skip the vertical dimension, which is where most site failures actually occur.
What to do instead
Adopt a fixed checklist for any site-evaluation item: soil texture and structure, depth to bedrock or restrictive layer, depth to seasonal saturated zone (mottling and redox features as indicators), slope, setback distances, and available replacement area. When a question gives you only a texture, state that texture alone is insufficient without depth and water-table information.
Reporting a wet, odorous, or unusually green strip over the absorption area as 'just landscaping' or omitting it, instead of naming it as a surfacing/seepage failure indicator and recommending prompt evaluation.
Candidates are reluctant to call a defect without digging, and they fear over-reporting, so they soften the finding into a neutral observation.
What to do instead
Learn the standard failure-indicator cluster - surfacing effluent, ponding, sewage odor, lush striping over lines, backed-up fixtures, slow drains, gurgling, alarm on a pump system, and unusually frequent pumping - and write each as a finding with a cause and a next step. Practice phrasing: 'Effluent surfacing at the absorption area indicates the system is not accepting flow; recommend evaluation by a qualified septic professional before further use.'
Performing or recommending destructive probing, opening the tank, or entering the tank or drainfield area during a general inspection, and then describing the result as if a full septic evaluation occurred.
Candidates want to be thorough and demonstrate expertise, and they conflate a visual/non-invasive inspection with a specialized septic evaluation.
What to do instead
Separate the two scopes explicitly: a home inspection is non-invasive and visual, may include locating and inspecting accessible components and the absorption area surface, and does not include opening tanks, pumping, probing soil, or camera inspection unless separately contracted. State the limitation in the report and recommend a licensed septic contractor for full evaluation.
Entering or reaching into a septic tank, or standing at an open tank lid, without recognizing the confined-space, toxic-gas, and fall hazards, and answering safety items as if the main risk were odor nuisance.
The tank looks like a shallow pit, and the real killers - hydrogen sulfide, methane, oxygen deficiency, and the open lid itself - are invisible, so hazard salience is low.
What to do instead
Memorize the safety hierarchy: never enter a tank (confined space, permit-required), keep lids secure and traffic-rated, keep people and equipment back from open covers, ventilate and stay upwind, and treat any open or damaged cover as an immediate hazard requiring barricade and correction. Answer safety items by naming the specific gas or fall hazard, not 'bad smell'.
Recommending additives, 'shock' treatments, or chemical/biological boosters as a fix for a sluggish system, rather than identifying the underlying cause such as excessive solids, hydraulic overload, or a clogged biomat.
Additive marketing is ubiquitous and candidates absorb the idea that a product can restore a failing drainfield.
What to do instead
Learn the position to defend: additives are not a substitute for maintenance and do not restore a failed absorption area; the correct response to slow drainage is to identify the cause (solids carryover, water use, biomat, roots, broken pipe) and recommend professional evaluation and, where appropriate, pumping or restoration. Practice rejecting additive questions by naming the mechanism.
Advising homeowners to pump on a fixed calendar schedule (for example, 'every three years') without referencing tank size, household occupancy, and solids accumulation, and without explaining what else maintenance includes.
Simplified rules of thumb circulate widely and are easy to memorize, so candidates repeat them as universal truths.
What to do instead
Answer with the variables: pumping interval depends on tank capacity, number of occupants, water use, and garbage disposal use, and is best set by measuring scum and sludge thickness against the clear-space zone. Pair it with the rest of the maintenance message - protect the drainfield from vehicles and roof drainage, keep inappropriate items out, conserve water, know the location of components, and keep records.
Failing to distinguish a conventional gravity system from an alternative (mound, sand filter, aerobic treatment unit, pressure-dosed, or pump-to-community) system, and applying conventional maintenance expectations to a system that needs power, service contracts, or more frequent attention.
Most study material centers on the conventional tank-and-drainfield, and candidates do not recognize the visual and functional cues of alternative systems on site.
What to do instead
Build a recognition list: a mound above grade, a blower or control box, alarms, risers and inspection ports, a dosing tank, or a pump chamber each signal a system requiring power, service, and specific maintenance. When a question presents these cues, answer with the alternative-system maintenance and inspection expectations rather than the conventional ones.
Writing vague report language such as 'septic system appears functional' or 'septic system has issues', without stating what was observed, what was not inspected, and the specific recommendation and its urgency.
Candidates worry about liability and hedge into meaningless statements, or they list a defect without the actionable next step.
What to do instead
Use a three-part finding template: observation (what you saw and where), implication (what it means for the system), and recommendation (who should do what, and how soon). Include the scope limitation - components not located, not opened, not tested - so the report is accurate and defensible. Rewrite weak sentences into this template as practice.